by William M. Welch and John Bacon, USA TODAY

by William M. Welch and John Bacon, USA TODAY

Search crews pursuing electronic pings that could be from the missing Malaysian jetliner's black boxes have failed to relocate the sounds heard deep in the Indian Ocean, the head of search operations said during a press conference Tuesday.

Sound locating equipment has picked up no trace of the signals since they were first heard late Saturday and early Sunday, said Angus Houston, who is heading the search off Australia's coast.

During the press conference in Perth, Australia, minister of defense David Johnston said crews would deploy the submersible vessel when they get another ping transmission.

"This is the most positive lead we've had and as I've said, we are pursuing it aggressively," Johnston said.

Warren Truss, Australia's acting prime minister said Tuesday in Perth that searchers were frustrated Monday in trying to re-connect with the underwater signals.

"Today is another critical day as we try and reconnect with the signals that perhaps have been emanating from the black box flight recorder of the MH370," Truss said.

"The connections two days ago were obviously a time of great hope that there had been a significant breakthrough and it was disappointing that we were unable to repeat that experience yesterday."

Truss said the crew on board the Australian ship Ocean Shield will launch the underwater vehicle, the Bluefin 21 unmanned sub, Tuesday. The craft is loaded with equipment that can create a sonar map of the area to chart any debris on the sea floor.

Houston said Monday that the equipment on board the Ocean Shield heard two distinct sounds late Saturday and early Sunday that were consistent with an aircraft's voice and data recorders.

An intense air and sea search was underway in the Indian Ocean, with 14 planes and 14 ships operating in good weather conditions off Australia's western coast, search coordinators said in a statement issued Tuesday morning in Perth.

Efforts were focused on a single search area nearly 30,000 square miles in size, they said.

Houston warned Monday that it could be days before authorities determine whether the signals came from the cockpit voice and data recorders onboard Malyasian Airlines Flight 370, which was lost a month ago after departing Kuala Lumpur for Beijing.

"In very deep oceanic waters, nothing happens fast," Houston said.

The signal was detected for two hours and 20 minutes late Saturday, then for 13 minutes early Sunday, Australian time. No further findings were reported as of Monday night.

Searchers were pressing to determine the exact position of the signal picked up about 1,000 miles off the coast of Perth, Australia. If they can pinpoint the location, an unmanned miniature submarine can be sent deep into the Indian Ocean to try to identify wreckage and find the two black boxes.

The difficult search for the boxes could lead to changes. The International Air Transport Association, whose members include most of the world's airlines, has created a task force to find ways to ensure that commercial jets are constantly tracked.

"We must never let an aircraft go missing in this way again," IATA director Tony Tyler said Monday at a conference in Abu Dhabi, United Arab Emirates. "Cooperative efforts continue on an unprecedented scale to find the aircraft, understand what happened and make sure that it does not happen again."

Authorities hope information obtained from Flight 370's boxes will help explain the mysteries of the Beijing-bound Boeing 777 that disappeared March 8 with 239 people aboard after taking off from Kuala Lumpur, Malaysia. The last, best analysis of data from several sources put the plane somewhere in the southern Indian Ocean.

"Clearly this is a most promising lead, and probably in the search so far, it's probably the best information that we have had,'' Houston said of the signals. "I'm much more optimistic than I was a week ago."

The obstacles are many. The first one is time: The batteries in the boxes that power the signals are only designed to last about 30 days or so.

Then there is the matter of the deep, vast Indian Ocean, a body of water covering more than 25 million square miles with an average depth of more than 2½ miles. The uncertain flight path of the missing jet had pushed the search area to almost 100,000 square miles.

The pings have narrowed the search area considerably, but the math remains daunting.

The Australian navy vessel Ocean Shield picked up the signals using a U.S. Navy device called a towed pinger locator, which is pulled behind the ship at a depth of about 2 miles.

The pinger locator is designed to detect signals at a range of 1 mile, meaning it would need to be almost on top of the black boxes to detect them if they were on the ocean floor.

The Ocean Shield is continuing to comb the ocean, trying to find the signal again.

"We are now in a very well defined search area, which hopefully will eventually yield the information that we need to say that MH370 might have entered the water just here," Houston said.

Adding to the struggle, a Chinese search vessel, Haixun 01, also said it briefly heard signals over the weekend. Those signals, more than 300 miles from the other signals, were being investigated by a British naval vessel, HMS Echo, which also has sophisticated listening technology.